From The Link Reaction To The Krebs Cycle
- In the link reaction (covered in C1.2.11), pyruvate is oxidized and decarboxylated to produce acetyl-CoA.
- Acetyl-CoA carries a 2-carbon acetyl group that still contains significant chemical energy.
- The Krebs cycle is where this acetyl group is completely broken down to extract that energy.
Definition
The Krebs cycle
The Krebs cycle (also known as the citric acid cycle) occurs in the mitochondrial matrix and is a key part of aerobic respiration.
Step-by-Step: How the Krebs Cycle Works
Step 1: Acetyl Group Enters the Cycle
- The acetyl group (2C) from acetyl-CoA is transferred to a 4-carbon molecule called oxaloacetate.
- This forms a 6-carbon molecule called citrate.
- Coenzyme A is released and recycled back to the link reaction.
2C (acetyl) + 4C (oxaloacetate) = 6C (citrate)
Exam techniqueYou only need to remember two molecules for the exam: citrate (6C) and oxaloacetate (4C).
Step 2: Removing Hydrogens (Oxidation/Dehydrogenation)
- As citrate moves through the cycle, hydrogen atoms are systematically removed.
- Each hydrogen atom consists of a proton (H⁺) and an electron (e⁻).
- These hydrogens are transferred to carrier molecules:
- NAD⁺ accepts hydrogens and becomes NADH (happens 3 times).
- FAD accepts hydrogens and becomes FADH₂ (happens 1 time).
Total oxidations per cycle: 4 (3 produce NADH, 1 produces FADH₂)
Step 3: Removing Carbons (Decarboxylation)
- The cycle removes carbon atoms as carbon dioxide (CO₂).
